1. Field of the Invention
The present invention relates to a storage device used in a computer system.
2. Related Background Art
In a conventional system, a high-speed storage device and a low-speed storage device may be connected to a computer called a hierarchical storage. In the system, files that are frequently used are stored in the high-speed storage device such as a magnetic disk device, while files that are not frequently used are stored in the inexpensive, low-speed storage device such as a tape device. Which files should be placed, i.e., stored, in which storage device is determined by using a table that manages access frequency of each file.
In another conventional system, a plurality of logical storage devices having different processing speeds and storage capacities are configured within a storage device that is connected to a computer and used. Such a system may be represented by disk array subsystems. In this system, the storage device manages as statistical information the frequency of accesses from the computer to data stored in the storage device, and, based on the statistical information, transfers data with high access frequency to logical storage devices with higher performance.
The first set of problems entailed in the prior art is that there is a high dependency on the computer connected to the storage device, that there is a limitation in the system configuration, and that it is difficult to simplify the system management.
In the conventional system described above, a hierarchical storage control is realized through software operating on the computer. The hierarchical storage control refers to a data storage control for controlling a plurality of storage regions having different processing speeds and storage capacities such that the storage regions can be changed according to the frequency of data usage. In other words, the hierarchical storage control refers to controlling to select, based on the property of data such as frequency of data usage, an appropriate storage region from among a plurality of storage regions having different properties in terms of processing speed and/or storage capacity, and to store the data in the storage region selected. However, when the system configuration is altered, such as when an old computer is replaced by a new computer, maintaining the system can be difficult due to such reasons as the system configuration of the new computer not being able to take over the software's control information.
Also in the conventional system described above, although a hierarchical storage control is implemented on a per-logical storage device basis, a technology for the storage device to recognize a data structure of data stored in the logical storage device or a technology for executing exclusive control are not disclosed. As a result, it would be difficult for a plurality of computers to share the same logical storage devices, and integrating storage devices used by a plurality of computers in order to reduce the management cost of the computer system would require imposing certain limitations on the configuration of the computer system, such as allocating a logical storage device for each computer.
The second problem is that optimal placement of data according to the life cycle or type of data is difficult.
According to the conventional technology, data that had high access frequency in the past is assumed to have high access frequency in the future as well, and the storage regions in which the data is stored are determined based on statistical information regarding data access frequency and on used capacity of storage regions that can be accessed at high-speed. The processing efficiency can be improved by increasing the probability with which data with high access frequency can reside in a storage device that can be accessed at high-speed. However, there are no technologies disclosed for determining storage regions in which to store data by taking into consideration differences in data properties that are dependent on the data's life cycle stage, i.e., the time elapsed since the corresponding file was generated, the type of application that generates and uses the data, and the type of data itself.
The third problem is that the effect of the hierarchical storage control is small.
Although the conventional system described above executes a hierarchical storage control by taking advantage of the difference in capacity and price between magnetic tapes and magnetic disks, the difference in capacity and price between magnetic tapes and magnetic disks have been growing smaller in recent years; consequently, the effect of cost optimization and cost reduction through the use of hierarchical storage control has also been growing smaller. Furthermore, due to the fact that the access speed to magnetic tapes is extremely slow compared to access speed to magnetic disks, it is difficult to use magnetic tapes as storage device for online access.
In the conventional system described above, a hierarchical storage control is executed by taking advantage of the difference in price and performance resulting from different RAID configurations of magnetic disks; however, since the price difference results only from the difference in the degree of redundancy in RAID configurations, the only cost reduction that can be hoped for is the cost reduction equivalent only to the difference in the degree of redundancy.